The present invention relates to a method for separating a substance by recrystallization, and more particularly to a method for separating a desired substance from a solid mixture by repeated recrystallization and an apparatus suitable for use in practicing the method.
Separation and purification of a substance by crystallization have been widely conducted in the chemical industry. Particularly, crystallization is an effective separation means for a substance to which a separation means utilizing vapor-liquid equilibrium such as distillation is hard to apply due to its small vapor pressure. However, there are often cases in which no sufficient separation is made by conducting crystallization one time. In such cases, a method in which recrystallization is repeated is commonly used in laboratories to obtain a desired substance having a desired purity, but this method is hard to adapt industrially due to low yield of separation.
The yield can be improved by utilizing a filtrate obtained in recrystallization for a substance having a higher purity as a solvent for crystallization of the substance having a lower purity in a prior stage, and a multi-stage separation and crystallization method based on this idea is proposed. The principle of the multi-stage separation and crystallization method is well known, and may be represented by the diagram in FIG. 1 in which:
1 to n: Number of the recrystallization stage PA1 Z: Raw material PA1 C: Crystals PA1 M: Solute in recrystallization filtrate PA1 L: Solvent
In this method, however, solid and liquid phases are handled, and this complicates the procedure and prevents the method from being industrially utilized.
As a method which does not require the procedure of separating solid and liquid phases, there is proposed a multi-stage separation and crystallization method in which crystals are deposited on the wall of a crystallization tower serving as a crystallizer and the mother liquor of crystallization is employed as a solvent for crystallization in another crystallizer of the prior stage, as disclosed in Japanese Examined Patent Publication (Tokkyo Kokoku) No. 41637/1978. However, this method has drawbacks in that the efficiency is low and the work is complicated, because it is a complete batch system using a single crystallizer in each stage.
Accordingly, it is an object of the present invention to provide a method for separting a desired substance in high yields by multi-stage recrystallization which can be efficiently conducted in a simple manner.
A further object of the invention is to provide a method for separation or purification by recrystallization which is conducted in a continuous and repeated mode in a single crystallizer.
Another object of the invention is to provide a crystallizer which is simple in structure and is suitable for practicing the method of the invention.
These and other objects of the present invention will become apparent from the description hereinafter.